The present invention relates to magnetic resonance imaging ("MRI") and, more particularly, to a method and an apparatus for selectively enabling coils in an MRI host device.
Initially, MRI systems used whole body coils to image subjects, such as human patients. The whole body receive coils of these systems had the advantage that sensitivity was, to a first approximation, substantially constant over the entire region being imaged. While this uniformity in sensitivity was not strictly characteristic of such whole body receive coils, the sensitivity was substantially constant to a degree that most reconstruction techniques assumed a constant coil sensitivity. Due to their large volume, however, the whole body receive coils suffer from a relative insensitivity to, individual spins.
For certain applications, a surface coil is preferable to a whole body receive coil in MRI systems. For an example of a surface receiving coil, see U.S. Pat. No. 4,793,356 to Misic et al. Surface coils can be made much smaller in geometry than whole body receive coils, and for medical diagnostic use they can be applied near, on, or inside the body of a patient. This is especially important where attention is directed to imaging a small region within the patient, rather than an entire anatomical cross section. The use of a surface coil in MRI systems also reduces the noise contribution from electrical losses in the body in comparison to a corresponding whole body receive coil, while maximizing the desired signal. MRI systems thus typically use small surface coils for localized high resolution imaging.
A disadvantage of surface coils, however, is their limited field of view. A single surface coil can only effectively image a region of a subject having lateral dimensions comparable to the surface coil diameter. Therefore, surface coils necessarily restrict the field of view, and inevitably lead to a tradeoff between resolution and field of view. Generally, large surface coils generate more noise due to their exposure to greater patient sample losses and therefore have a larger noise component relative to the signal, while smaller coils have lower noise but in turn restrict the field of view to a smaller region.
One technique for extending the field-of-view limitation of a surface coil is described in U.S. Pat. No. 4,825,162, entitled "Nuclear Magnetic Resonance (NMR) Imaging with Multiple Surface Coils," issued to Roemer et al. Roemer et al. describes a set of surface coils arrayed with overlapping fields of view. Each of the surface coils is positioned to have substantially no interaction with any adjacent surface coils. A different response signal is received at each different surface coil from an associated portion of the sample that was enclosed within an imaging volume defined by the array. Each different response signal is used to construct a different one of a like plurality of different images of the sample. The different images are then combined to produce a single composite image of the sample. Roemer et al. describes a four coil array for imaging a human spine.
While an increased number of surface coils can be used in this manner to increase the field of view of MRI systems, MRI system scanners typically have a limited number of simultaneous data acquisition channels or receivers, and a limited number of selectable inputs. The number of selectable inputs is typically equal to the number of receivers. In some cases the number of selectable inputs is double the number of receivers, each receiver being capable of selectively receiving from either of two inputs. The number of data acquisition channels and separate inputs is therefore a design limitation on the number of phased array surface coils that can be used in an MRI system. A disadvantage of conventional phased array surface coils, therefore, is that the surface coil array can include only the number of surface coils that can be directly connected to the phased array inputs of the system scanner. The number of simultaneous data acquisition channels, or receivers, can be a further restriction on the utility of surface coil arrays.
To overcome the limitations of MRI system scanners imposed by the limited number of data acquisition channels or receivers, and the limited number of inputs, MRI technicians have resorted to physically moving the surface coils or manually switching selected groups of coils after successive scans to obtain MRI images. As can be appreciated, these techniques require excessive scan room intervention by personnel operating the MRI systems. That is, after each scan a technician must enter the scan room to physically reposition the coils, or manipulate a local selector switch to reconfigure the active coils of a large array to those needed to cover the desired patient anatomy. These scan room intervention techniques increase examination time and the likelihood of a patient rejecting the procedure.